Bonding of lead wires to a microcircuit chip mounted on a lead frame for coupling to external circuitry is generally accomplished by ball bonding. According to this technique, a lead wire or bonding wire is held in a capillary tool so that the lead wire projects beyond the end of the tool. The capillary tool forms part of a ball bonding machine in which the tool is appropriately mounted and positioned over the metalized die pad of an integrated circuit chip. A ball is formed at the end of the lead wire or bonding wire by, for example, arc discharge between the bonding wire and another electrode. After solidification, the metal ball at the end of the lead wire is brought into intimate contact with the metalized die pad and a bond is formed typically by thermocompression or ultrasonic bonding.
Prior art methods of ball formation for bonding lead wires generally suffer from lack of control over the ball formation process with subsequent variation in the size, shape, overall strength and quality of the bonding ball. In particular, one of the problems associated with prior art methods of ball formation is the thinning and weakening of the lead wire which often occurs above the bonding ball. Such thinning and weakening of the lead wire at the stem adjacent to the ball is referred to as "necking" and presents the risk of failure or fracture of the lead wire at the ball bonding site. Further disadvantages of prior art ball bonding machines and methods are discussed in the "Background Art" section of U.S. Pat. No. 4,390,771 referred to above.